The present invention relates to an improved method and apparatus for bending elongate workpieces, particularly pipes. More specifically, the present invention relates to an improved method and apparatus for bending elongate workpieces, particularly pipes, of the type wherein a bending moment is applied to the workpiece while inductively heating a cross-sectional zone of the workpiece, by means of an induction loop which surrounds the cross-sectional. zone, such that a non-uniform temperature distribution along the circumference of the cross-sectional zone results, and wherein the bend workpiece is subsequently cooled. In addition to pipes, the inductively heatable and thus electrically conductive elongate workpieces employed also include rods, in particular, and, more generally, all those elongate pieces which can be bent by means of nonuniform temperature distribution along the circumference of a heated, usually narrow circumferential zone.
Nonuniform heating of a cross-section, for example, of pipes to be bent, and more generally, of elongate workpieces to be bent, is of great significance in some special cases and additionally, from a general point of view, results in special advantages. There are materials which tend to crack when subjected to extensive plastic deformation, particularly elongation, if such deformation takes place at temperatures above the so-called Ac3 point, being the temperature at which the material structure changes into an austenitic phase (approximately 800.degree. to 850.degree. C. for steel). If now, while a bend is being made, the temperature at the side being elongated or stretched, the so-called extrados, is kept low, e.g. at 750.degree. C., and at the the so-called intrados, it is kept high, e.g. at 950.degree. C., the neutral bending line is displaced toward the extrados. This results in more upsetting at the intrados and less stretching or elongation at the extrados. Less elongation at the extrados is advantageous not only because it decreases the danger of crack formation in that area, but also because less elongation means less thinning of the pipe wall. By using the correct temperature difference between extrados and intrados, it is possible, starting with pipes having normal wall thicknesses, to produce bends which meet the conditions of maximum permissible wall thinning (at the extrados) and wall thickening (at the intrados), as prescribed, for example, in DIN (German Industrial Standard) 2413.
Heating of the elongate workpieces, e.g. pipes, by means of an inductor to produce progressive bending is widely known. For example, the general aspect of such heating is disclosed in Dutch Pat. No. 142,607 and Federal Republic of Germany Pat. No. 2,112,019.
A first possibility for nonuniform heating by means of an inductor to produce progressive bending in pipes and other elongate workpieces with the aim of influencing or correcting the degree of deformation and change in wall thickness is disclosed in German Pat. No. 2,738,394 which provides for asymmetrical placement of the inductor. One of several alternative possibilities disclosed in DE-OS (Federal Republic of Germany Laid-Open Patent Application) No. 2,220,910 has similar aims, and provides that the portion of the inductor surrounding the extrados has a greater distance from the pipe surface than the portion surrounding the intrados.
For the same purpose, DE-OS No. 2,220,910 discloses a second alternative according to which nonuniform or asymmetrical heating is realized by means of sheet metal packets arranged locally along the circumference of the inductor so as to serve as yokes with which the intensity of the inductive heating can be influenced. A third, complicated and spatially difficult to manage alternative is disclosed in DE-OS No. 2,220,910 which provides that, in conjunction with such metal yokes, a first inductor is provided for preheating and a second inductor is provided for primary heating at the heating cross section.
For the same purpose, U.S. Pat. No. 4,177,661 discloses an inductor in which part of the induced energy is collected by a shield placed between the inductor and the pipe to be bent. This patent also describes an apparatus according to which an additional heating source is provided at the intrados upstream of the annular, inductively operating heating source.
However, all these known methods and apparatuses for nonuniform heating ofthe bending cross section of pipes or other elongate workpieces to be bent in a progressive manner have certain drawbacks and do not bring success or are very difficult to implement successfully.
Asymmetrical heating by means of eccentric placement or radial displacement of the inductor has the drawback that the transverse displacement of the inductor for the purpose of reducing the temperature of the extrados simultaneously results in an increase in the temperature of the intrados, although it would be preferable to change the temperature primarily only at the extrados in order to avoid undesirable wall thinning, e.g. of a pipe, at the extrados, and excessive upsetting of the intrados is likewise undesirable. Moreover, the inductors employed usually have spray holes which spray water under a certain angle in the forward direction onto the already bent pipe so as to delimit the heated zone. Transverse displacement of the inductor also changes the point at which the water jets impinge on the pipe, with the result that the width of the heated zone becomes nonuniform and thus has an adverse influence on the bending process.
The attachment of metal yokes to the inductor at the point on the inductor where a higher temperature is desired is a very complicated and time-consuming job. If, finally, the desired effect has been produced for a certain pipe wall and for the intended bending radius, the specific inductor employed is suitable only for this particular case.
The attachment of a shield between the inductor and the workpiece to be heated is difficult to accomplish in practice since the gap available between the inductor and the workpiece is small. Additionally, the shield must also be cooled with water to prevent overheating, so that practically no space is left. Here again, the end effect is difficult to predict and the position and dimensions must be determined empirically, with success being applicable only to one specific case.
The local placement of an additional heat source for the purpose of producing a higher temperature at the inner arc, i.e. at the intrados, has the drawback that the heating zone is made extra wide at that point. This is a drawback for retention of a good round pipe during bending and may also cause dents or local upsets.
The present invention is based on the recognition in the field of the invention of asymmetrically inductively heating the workpiece by placing the inductor eccentrically according to the acknowledged prior art (e.g. according to German Pat. No. 2,738,394) or, if inductor placement is concentric, on partially shielding the induction (U.S. Pat. No. 4,177,661). The former case, a priori, does not permit a concentric inductor arrangement, while the latter case is complicated and difficult to realize. The field of the invention also includes the arrangement employing metal yokes according to DE-OS No. 2,220,910 which, however, can be adjusted accurately, and in a very complicated manner, only to one particular workpiece at a time.